1. Field of the Invention
The present invention relates to the fabrication of rigid thin windows for vacuum applications, such as electron guns or x-ray detectors. In particular, the windows are made using photolithographic and wet chemical etching techniques.
2. Description of Related Art Thin film windows are used in applications where it is necessary to separate a vacuum environment from another environment, such as ambient atmosphere, a gaseous environment, or a liquid. These vacuum applications include electron and ion guns, x-ray tubes, x-ray detectors, and chambers transmitting other electromagnetic radiation or charged or neutral particles. The window must be able to withstand the pressure difference and possibly a high temperature environment, while not significantly impeding the beam of particles or radiation passing through the window.
An important application of thin film windows is in electron beams, which are used in industry for rapid curing of inks and paint, surface treatment of paper products, and destruction of organic waste solvents. The electron beam tubes typically used in these applications have electron windows made of 15 micron thick titanium foil. The electron window must be thin enough to allow electrons to pass through, be gas impermeable, and have sufficient strength to stand off an atmosphere of pressure drop. These guns typically operate in the 150 to 175 kilovolt range.
Recent advances in electron gun design allow for the operation of electron guns at much lower voltages. These improved electron guns do not require pumping and are designed to efficiently produce an electron beam in air with voltages below 100 kilovolts and as low as 25 kilovolts. An integral part of this electron gun is the thin, several micron thick window that stands off atmospheric pressure (1 atm) and permits electrons generated by the electron source to pass from the evacuated side to the outside. The window must be capable of transmitting electron current densities of several milliamperes per square centimeter into air with 90% efficiencies at 50 kilovolts for thousands of hours of operation. In addition, the window must also survive a high temperature bonding operation for attachment to the electron tube.
A method of making an electron beam window is discussed in U.S. Pat. No. 4,468,282 issued Aug. 28, 1984 to Neukermans. The electron permeable windows are fabricated by depositing a single layer of an inert, high strength, low atomic number material onto a substrate. The window pattern is photolithographically defined and the substrate is etched to leave the window structure. Neukermans produces a simple single layer window that is susceptible to breakage throughout the manufacturing and bonding process.
An improved window structure and composition has been developed which significantly increases window durability and allows the use of the windows in a wide variety of environments, including very corrosive ones. The present invention produces a micromachined bi-material window with a rigid support frame structure, which has demonstrated impressive electron gun performance and improved window survivability during window bonding operations.